1. Field of the Invention
The invention relates to electrical machines of the kind having two relatively rotatable components with respective opposed surfaces separated by a gap into which a surrounding fluid may flow during operation of the machine. Examples of such machines are electric motors, generators and torquer-generators comprising a relatively rotatable armature and magnetic pole structure separated by a gap, which is usually annular, but the invention is generally applicable to any other type of electrical machine having spaced relatively rotatable surfaces.
2. Description of Related Art
In circumstances where the electrical machine is required to operate within a surrounding fluid, it may be advantageous or necessary for the fluid to flow through the gap between the relatively rotatable surfaces. For example, the fluid may be used to lubricate bearings supporting the relatively rotatable components or to cool one or both of the components. Alternatively the circumstances and construction may be such that it is difficult or impossible to provide a rotating seal which will reliably prevent flow of the fluid into the gap. In these circumstances the problem may arise that material entrained in the fluid, and especially magnetic material, may become deposited within the gap as the fluid flows through it and such deposit may increase to an extent where it interferes with, or even prevents, the effective operation of the machine.
For example, British Patent Specification No. 2257182 describes a roll stabilised instrument package system for use down a borehole in the earth for controlling a rotary drilling system. The system comprises an instrument carrier which is mounted within a drill collar for rotation about the longitudinal axis of the collar. An impeller is rotatably mounted in bearings on the instrument carrier and carries a pole structure which co-operates across an annular gap with an armature within the instrument carrier, so as to act as a torquer-generator. During operation drilling fluid flows along the drill collar and around the instrumentation package system. The impeller rotates the carrier relative to the drill collar as a result of the flow of drilling fluid through the impeller and the torque transmitted by the impeller to the instrument carrier is controlled so as to maintain the carrier nonrotating in space and hence roll stabilised, or to rotate the carrier in a required manner for other control purposes.
The bearings between the impeller and instrument carrier may be lubricated by the drilling fluid which flows into the annular gap between the impeller and the instrument carrier. The drilling fluid, or mud, normally comprises fine powder particles in suspension in a liquid, which is usually water or an invert oil emulsion in water. In the course of drilling, the drilling fluid carries cuttings from the formation being drilled upwardly to the surface through the annulus between the drill string and the walls of the borehole. At the surface most of the formation material is removed from the drilling mud by shale shakers and hydrocyclones before the mud is pumped back down the hole through the drill string. However, the filtering processes are not perfect and some fine solid particles from the formation become entrained in the drilling fluid.
In drilling some types of formation, such as formations including magnetite, a proportion of the formation particles entrained in the drilling fluid may be magnetic. Also, although the basic powder material in suspension in the drilling fluid is usually nominally non-magnetic, it is commonly found that such material contains magnetic impurities. There is a tendency for any such magnetic particles to become deposited on the internal surface of the pole structure as the fluid passes through the gap between the pole structure and the instrument carrier, as a result of the magnetic attraction of the pole structure. The present invention provides an arrangement whereby in these, or similar circumstances in other electrical machines, the build-up of such deposited particles may be prevented from interfering significantly with the operation of the electrical machine.